How the bow shock does it

Abstract

Between the world of living organisms and inanimate objects exists a zone inhabited by animate, but nonliving, physical entities. These fascinating, and often nonlinear systems exhibit characteristics which in many ways mimic that of simple biological organisms. They seem to have an instinctive desire to accomplish certain tasks, and devise many creative strategies to achieve their goal. The bow shock, the topic of this paper, is one of the inhabitants of this zone and it forms due to solar wind interaction with the Earth's magnetic field. As one would expect, its goal is to decelerate and divert the supersonic solar wind flow around the magnetosphere. But unlike the usual shock waves, it has to accomplish the task without the use of collisions; quite a challenge indeed. A bigger challenge, however, has been for us to use space age technology and supercomputers to find out how the bow shock does it! We have made much progress; many of the pixels are in place but the picture is not complete. What we have found is a truly fascinating and complex story which is a testament to the creativeness of the bow shock. Depending on the upstream Mach number (ratio of the solar wind speed to the sound speed) and location on the paraboloid surface of the shock, a different strategy for dissipation of the flow energy into heat is utilized. In some regimes, the transition from upstream to downstream takes place on short ( ∼100 km) length scales, while in others, the shock transition is masked with an extended region of electromagnetic waves and turbulence. As part of the dissipation process, electrons and ions are reflected off the shock. The interaction between the solar wind and the reflected particles forms the foreshock, a region extending many Earth radii upstream of the shock. The foreshock is populated by a variety of electrostatic and electromagnetic waves, making it a great natural laboratory for studies of nonlinear wave‐particle interactions. Among these are the fast magnetosonic waves which steepen to form spatially localized shock waves, named shocklets! Creation of shocklets by the bow shock is not quite reproduction, but is close enough to stir the imagination and make one marvel at its parallelism to live organisms. There are many motivations for the study of the bow shock. One set deals with the inherent interest in collisionless shocks,and how it can be used to enhance our knowledge of nonlinear plasma physics, collisionless dissipation, particle acceleration, and their extension to astrophysical settings. The other concerns the influence of the bow shock on magnetospheric phenomena. Before reaching the magnetopause, the solar wind is greatly modified by the bow shock through heating, deceleration, and considerable enhancement of its field fluctuations.